Abstract: A process for the preparation of a solid catalyst of the Ziegler-Natta type comprises precipitating the solid catalyst in a liquid hydrocarbon medium by reacting (A) a solution of a soluble magnesium alkoxide and (B) a transition metal halide which is a halide of Ti (IV), a halide of V (IV) or a halide of VO (III) in the presence of (C) at least one transition metal alkoxide which is free from halogen and soluble in liquid hydrocarbon. The precipitated solid catalyst consists of spheroidal particles having a mean diameter by mass of from 10 to 70 microns and a narrow particle size distribution such that the ratio of mean diameter by mass to the mean diameter by number is greater than 1.2 and smaller than 2.0.